Unlike the three creatine kinase (CK) isoenzymes, the clinical significance of the macro CK forms is poorly understood. Therefore, we analyzed, in 136 patients identified as having macro CK, the relation between the macro CK isoenzymes types 1 and 2 and other laboratory and clinical data. We found characteristic laboratory patterns (e.g.,the presence of elevated total CK and CK-MB, and the size of the macro CK fraction compared to total CK) and clinical settings for the occurrence of the two different macro CK forms. In particular, patients with macro CK type 1 (a complex of CK-BB and immunoglobulin) most often had myositis, whereas those with macro CK type 2 most commonly had a malignancy. The findings indicated that the presence of macro CK isoenzymes has a low prognostic value for impending death, but may support the diagnosis of an autoimmune process (type 1) or malignant cell proliferation (type 2). Thus, the recognition and proper interpretation of these unusual ("atypical") CK isoenzyme patterns is important for clinical practice. Laboratory textbooks usually warn against unlidded centrifugation because of the health hazards associated with aerosol production, but often fail to mention the possibility of water loss. We observed unexpectedly high analyte concentrations in specimens which, due to hyperlipidemia, have been "clarified" by ultracentrifugation and demonstrated that the phenomenon occurred due to the lack of capping of centrifuge tubes. The findings re-emphasize that regular and thorough monitoring of specimen processing should be an integral part of good clinical laboratory practice. Blood specimens received in the laboratory in clot tubes intended to yield defibrinated serum for analysis do not always clot fully and/or promptly. In evaluating an automated immunoassay system (A1A1200, Tosoh), we observed some spurious results, e.g., for follitropin (FSH) with a frequency of equal to or less than 1%. We showed that the spurious results were caused by incomplete clotting due to contamination of the blood sample with anticoagulants such as heparin and ACD or due to clotting factor deficiency as evidenced by finding prolonged prothrombin time in some patients. The spurious results could be eliminated by adding excess amounts of heparin that prevented postclotting in the assay receptable. The findings indicate that the consequences of incompletely clotted specimens should be considered when evaluating new methods and instruments.